Classes and Objects - Computer Science | myUSFapjoshi/cs112/notes/Classes_Objects.pdf ·...

Classes and Objects

Alark Joshi

Copyright © 2012 Pearson Education, Inc.

Announcements

• Grades for Quiz 1 and Quiz 2 are on Canvas

• Reading for Wednesday’s quiz – Section 3.1-3.8

Creating Objects

• A variable holds either a primitive value or a reference to an object

• A class name can be used as a type to declare an object reference variable

String title; • No object is created with this declaration


Creating Objects

• An object reference variable holds the address of an object

• The object itself must be created separately

• Generally, we use the new operator to create an object

Creating Objects

• Creating an object is called instantiation

• An object is an instance of a particular class


title = new String (“CS 112 is the best!");

This calls the String constructor, which is a special method that sets up the object

Invoking Methods • We've seen that once an object has been

instantiated, we can use the dot operator to invoke its methods

numChars = title.length()

• A method may return a value, which can be used in an assignment or expression

• A method invocation can be thought of as asking an object to perform a service


References • Note that a primitive variable contains the value

itself, but an object variable contains the address of the object

• An object reference can be thought of as a pointer to the location of the object

• Rather than dealing with arbitrary addresses, we often depict a reference graphically

"Steve Jobs" name1

num1 38


Assignment Revisited • The act of assignment takes a copy of a value

and stores it in a variable

• For primitive types: num1 38

num2 96 Before:

num2 = num1;

num1 38

num2 38 After:


Reference Assignment • For object references, assignment copies the

address:

name2 = name1;

name1

name2 Before:

"Steve Jobs"

"Steve Wozniak"

name1

name2 After:

"Steve Jobs"


Garbage Collection • When an object no longer has any valid references to

it, it can no longer be accessed by the program

• The object is useless, and therefore is called garbage

• Java performs automatic garbage collection periodically, returning an object's memory to the system for future use

• In other languages, the programmer is responsible for performing garbage collection


The String Class

• Because strings are so common, we don't have to use the new operator to create a String object

title = “CS 112 is the best!";

• This is special syntax that works only for strings

• Each string literal (enclosed in double quotes) represents a String object


String Methods

• Once a String object has been created, neither its value nor its length can be changed

• Therefore we say that an object of the String class is immutable


String Indexes • It is occasionally helpful to refer to a particular

character within a string

• This can be done by specifying the character's numeric index

• The indexes begin at zero in each string

• In the string "Hello", the character 'H' is at index 0 and the 'o' is at index 4

• See StringMutation.java



//******************************************************************** // StringMutation.java Author: Lewis/Loftus // // Demonstrates the use of the String class and its methods. //******************************************************************** public class StringMutation { //----------------------------------------------------------------- // Prints a string and various mutations of it. //----------------------------------------------------------------- public static void main (String[] args) { String phrase = "Change is inevitable"; String mutation1, mutation2, mutation3, mutation4; System.out.println ("Original string: \"" + phrase + "\""); System.out.println ("Length of string: " + phrase.length()); mutation1 = phrase.concat (", except from vending machines."); mutation2 = mutation1.toUpperCase(); mutation3 = mutation2.replace ('E', 'X'); mutation4 = mutation3.substring (3, 30); continued


continued // Print each mutated string System.out.println ("Mutation #1: " + mutation1); System.out.println ("Mutation #2: " + mutation2); System.out.println ("Mutation #3: " + mutation3); System.out.println ("Mutation #4: " + mutation4); System.out.println ("Mutated length: " + mutation4.length()); } }


continued // Print each mutated string System.out.println ("Mutation #1: " + mutation1); System.out.println ("Mutation #2: " + mutation2); System.out.println ("Mutation #3: " + mutation3); System.out.println ("Mutation #4: " + mutation4); System.out.println ("Mutated length: " + mutation4.length()); } }

Output Original string: "Change is inevitable" Length of string: 20 Mutation #1: Change is inevitable, except from vending machines. Mutation #2: CHANGE IS INEVITABLE, EXCEPT FROM VENDING MACHINES. Mutation #3: CHANGX IS INXVITABLX, XXCXPT FROM VXNDING MACHINXS. Mutation #4: NGX IS INXVITABLX, XXCXPT F Mutated length: 27

Quick Check


What output is produced by the following? String str = "Space, the final frontier."; System.out.println (str.length()); System.out.println (str.substring(7)); System.out.println (str.toUpperCase()); System.out.println (str.length());

Quick Check


What output is produced by the following? String str = "Space, the final frontier."; System.out.println (str.length()); System.out.println (str.substring(7)); System.out.println (str.toUpperCase()); System.out.println (str.length());

26 the final frontier. SPACE, THE FINAL FRONTIER. 26

Class Libraries

• A class library is a collection of classes that we can use when developing programs

• The Java standard class library is part of any Java development environment


Class Libraries • Its classes are not part of the Java language

per se, but we rely on them heavily

• Various classes we've already used

– System

– Scanner

– String

are part of the Java standard class library

The Java API

• The Java class library is sometimes referred to as the Java API

• API stands for Application Programming Interface

• Clusters of related classes are sometimes referred to as specific APIs:

– The Swing API – The Database API


The Java API

• Get comfortable navigating the online Java API documentation


Packages

• For purposes of accessing them, classes in the Java API are organized into packages

• These often overlap with specific APIs • Examples:


Package

java.lang java.applet java.awt javax.swing java.net java.util javax.xml.parsers

Purpose

General support Creating applets for the web Graphics and graphical user interfaces Additional graphics capabilities Network communication Utilities XML document processing

The import Declaration • When you want to use a class from a package, you

could use its fully qualified name java.util.Scanner

• Or you can import the class, and then use just the class name

import java.util.Scanner;

• To import all classes in a particular package, you can use the * wildcard character

import java.util.*;


The import Declaration • All classes of the java.lang package are imported

automatically into all programs

• It's as if all programs contain the following line:

import java.lang.*;

• That's why we didn't have to import the System or String classes explicitly in earlier programs

• The Scanner class, on the other hand, is part of the java.util package, and therefore must be imported


Questions?

String worksheet

The Random Class • The Random class is part of the java.util

package

• It provides methods that generate pseudorandom numbers

• A Random object performs complicated calculations based on a seed value to produce a stream of seemingly random values

• See RandomNumbers.java



//******************************************************************** // RandomNumbers.java Author: Lewis/Loftus // // Demonstrates the creation of pseudo-random numbers using the // Random class. //******************************************************************** import java.util.Random; public class RandomNumbers { //----------------------------------------------------------------- // Generates random numbers in various ranges. //----------------------------------------------------------------- public static void main (String[] args) { Random generator = new Random(); int num1; float num2; num1 = generator.nextInt(); System.out.println ("A random integer: " + num1); num1 = generator.nextInt(10); System.out.println ("From 0 to 9: " + num1); continued


continued num1 = generator.nextInt(10) + 1; System.out.println ("From 1 to 10: " + num1); num1 = generator.nextInt(15) + 20; System.out.println ("From 20 to 34: " + num1); num1 = generator.nextInt(20) - 10; System.out.println ("From -10 to 9: " + num1); num2 = generator.nextFloat(); System.out.println ("A random float (between 0-1): " + num2); num2 = generator.nextFloat() * 6; // 0.0 to 5.999999 num1 = (int)num2 + 1; System.out.println ("From 1 to 6: " + num1); } }


continued num1 = generator.nextInt(10) + 1; System.out.println ("From 1 to 10: " + num1); num1 = generator.nextInt(15) + 20; System.out.println ("From 20 to 34: " + num1); num1 = generator.nextInt(20) - 10; System.out.println ("From -10 to 9: " + num1); num2 = generator.nextFloat(); System.out.println ("A random float (between 0-1): " + num2); num2 = generator.nextFloat() * 6; // 0.0 to 5.999999 num1 = (int)num2 + 1; System.out.println ("From 1 to 6: " + num1); } }

Sample Run A random integer: 672981683 From 0 to 9: 0 From 1 to 10: 3 From 20 to 34: 30 From -10 to 9: -4 A random float (between 0-1): 0.18538326 From 1 to 6: 3

Quick Check


Given a Random object named gen, what range of values are produced by the following expressions?

gen.nextInt(25)

gen.nextInt(6) + 1

gen.nextInt(100) + 10


gen.nextInt(10) – 5


Quick Check


Given a Random object named gen, what range of values are produced by the following expressions?

gen.nextInt(25)

gen.nextInt(6) + 1





Range 0 to 24

1 to 6

10 to 109

100 to 149

-5 to 4

12 to 33

Quick Check


Write an expression that produces a random integer in the following ranges: Range

0 to 12

1 to 20

15 to 20

-10 to 0

Quick Check


Write an expression that produces a random integer in the following ranges:

gen.nextInt(13)

gen.nextInt(20) + 1

gen.nextInt(6) + 15


Range 0 to 12

1 to 20

15 to 20

-10 to 0

The Math Class • The Math class is part of the java.lang

package

• The Math class contains methods that perform various mathematical functions

• These include: – absolute value – square root – exponentiation – trigonometric functions


The Math Class • The methods of the Math class are static

methods (also called class methods)

• Static methods are invoked through the class name – no object of the Math class is needed value = Math.cos(90) + Math.sqrt(delta);

• We discuss static methods further in Chapter 7

• See Quadratic.java



//******************************************************************** // Quadratic.java Author: Lewis/Loftus // // Demonstrates the use of the Math class to perform a calculation // based on user input. //******************************************************************** import java.util.Scanner; public class Quadratic { //----------------------------------------------------------------- // Determines the roots of a quadratic equation. //----------------------------------------------------------------- public static void main (String[] args) { int a, b, c; // ax^2 + bx + c double discriminant, root1, root2; Scanner scan = new Scanner (System.in); System.out.print ("Enter the coefficient of x squared: "); a = scan.nextInt(); continued


continued System.out.print ("Enter the coefficient of x: "); b = scan.nextInt(); System.out.print ("Enter the constant: "); c = scan.nextInt(); // Use the quadratic formula to compute the roots. // Assumes a positive discriminant. discriminant = Math.pow(b, 2) - (4 * a * c); root1 = ((-1 * b) + Math.sqrt(discriminant)) / (2 * a); root2 = ((-1 * b) - Math.sqrt(discriminant)) / (2 * a); System.out.println ("Root #1: " + root1); System.out.println ("Root #2: " + root2); } }


continued System.out.print ("Enter the coefficient of x: "); b = scan.nextInt(); System.out.print ("Enter the constant: "); c = scan.nextInt(); // Use the quadratic formula to compute the roots. // Assumes a positive discriminant. discriminant = Math.pow(b, 2) - (4 * a * c); root1 = ((-1 * b) + Math.sqrt(discriminant)) / (2 * a); root2 = ((-1 * b) - Math.sqrt(discriminant)) / (2 * a); System.out.println ("Root #1: " + root1); System.out.println ("Root #2: " + root2); } }

Sample Run Enter the coefficient of x squared: 3 Enter the coefficient of x: 8 Enter the constant: 4 Root #1: -0.6666666666666666 Root #2: -2.0

Formatting Output • It is often necessary to format output values in certain ways so

that they can be presented properly

• The Java standard class library contains classes that provide formatting capabilities

• The NumberFormat class allows you to format values as currency or percentages

• The DecimalFormat class allows you to format values based on a pattern

• Both are part of the java.text package


Formatting Output • The NumberFormat class has static methods that

return a formatter object

getCurrencyInstance()

getPercentInstance()

• Each formatter object has a method called format that returns a string with the specified information in the appropriate format

• See Purchase.java



//******************************************************************** // Purchase.java Author: Lewis/Loftus // // Demonstrates the use of the NumberFormat class to format output. //******************************************************************** import java.util.Scanner; import java.text.NumberFormat; public class Purchase { //----------------------------------------------------------------- // Calculates the final price of a purchased item using values // entered by the user. //----------------------------------------------------------------- public static void main (String[] args) { final double TAX_RATE = 0.06; // 6% sales tax int quantity; double subtotal, tax, totalCost, unitPrice; Scanner scan = new Scanner (System.in); continued


continued NumberFormat fmt1 = NumberFormat.getCurrencyInstance(); NumberFormat fmt2 = NumberFormat.getPercentInstance(); System.out.print ("Enter the quantity: "); quantity = scan.nextInt(); System.out.print ("Enter the unit price: "); unitPrice = scan.nextDouble(); subtotal = quantity * unitPrice; tax = subtotal * TAX_RATE; totalCost = subtotal + tax; // Print output with appropriate formatting System.out.println ("Subtotal: " + fmt1.format(subtotal)); System.out.println ("Tax: " + fmt1.format(tax) + " at " + fmt2.format(TAX_RATE)); System.out.println ("Total: " + fmt1.format(totalCost)); } }


continued NumberFormat fmt1 = NumberFormat.getCurrencyInstance(); NumberFormat fmt2 = NumberFormat.getPercentInstance(); System.out.print ("Enter the quantity: "); quantity = scan.nextInt(); System.out.print ("Enter the unit price: "); unitPrice = scan.nextDouble(); subtotal = quantity * unitPrice; tax = subtotal * TAX_RATE; totalCost = subtotal + tax; // Print output with appropriate formatting System.out.println ("Subtotal: " + fmt1.format(subtotal)); System.out.println ("Tax: " + fmt1.format(tax) + " at " + fmt2.format(TAX_RATE)); System.out.println ("Total: " + fmt1.format(totalCost)); } }

Sample Run Enter the quantity: 5 Enter the unit price: 3.87 Subtotal: $19.35 Tax: $1.16 at 6% Total: $20.51

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